Multigrid electron discharge tube



MULTIGRI-D ELECTRON DISCHARGE TUBE .Filed Sept 12. 1939 OUTPUT INV EN TOR. DON 6. BURNS/0E ATTORNEY.

Patented Sept. 17, 1940 UNITED STATES PATENT OFFICE Don G. Burnside, East Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application September 12, 1939, Serial No. 294,414

6 Claims.

My invention relates to multigrid electron discharge tubes and more particularly to multigrid tubes of the type usually known as the variable mu or super-control pentodes.

6 In the conventional variable mu or super-control pentode having three grids between the oathode and anode, the No. 2 or screen grid is of uniform and of rather close mesh, the No. 3 or suppressor grid is of uniform and somewhat more open mesh so as to avoid over suppression, and

the No. 1 or control grid close to the cathode has one or more gaps or openings, or if of the helical type is of non-uniform pitch. At the usual operating bias the current density is greatest at the gaps or openings in the control grid, where the electron discharge forms a kind of electron beam.

The screen grid wires located in the path of the electron beam through the gap or opening in the control grid collect many electrons, often result- 0 ing in high screen grid current, while the balance of the electrons in the beam pass through the screen grid and reach the anode, resulting in a high plate current.

In a variable mu pentode it is ordinarily desirable to have a high ratio of control grid-plate transconductance to plate current and to screen grid current at the normal operating voltage, as a saving of plate and screen current is obtained. In some types of these tubes, particularly small tubes suitable for high frequency, such as those of the general type shown in U. S. patent to Salzberg, 2,017,549, October 15, 1935, there is in addition an improvement in useful life.

It has been found that a lower plate current and screen grid current can be obtained in variable mu pentodes by providing in the screen grid a gap which is opposite, and of about the same size as the gap in the control grid. It has also been found that with the gaps of the two grids opposite each other or aligned, the control grid has somewhat greater control of the plate current, and also that both the plate current and the screen current are reduced while the control grid-plate transconductance is maintained as a result of the improved control by the control grid.

For example, if a conventional variable mu pentode of the small high frequency type has an average plate current of 6 m. a. and an average control grid-plate transconductance of about 1670 micromhos, a similar tube, except for aligned gaps in the control grid and screen grid, may have an average plate current of 5.8 m. a., and an average control grid-plate transconductance of about 1840 micromhos. However, when these variable mu .pentodes with aligned gaps in the control grid and screen grid are placed in conventional circuits in which conventional variable mu or super-control pentodes operate satisfactorily, considerable feed-back and oscillation ocours.

The principal object of my invention is to provide a variable mu or super-control pentode which has a higher ratio of control grid-plate transconductance to plate current and also to screen grid current than the conventional variable mu 10 pentode and which can be used in conventional circuits without causing feed-back and producing oscillations.

Another object is to provide an improved variable mu pentode which has about the same grid 15 to plate capacitance as the conventional variable mu pentode and also a considerably higher ratio of control grid-plate transconductance to plate current.

In accordance with my invention the control .20 grid and the screen grid of a pentode have intermediate the ends of the grids gaps or more open portions in alignment radially of the cathode, and the shielding effect of the suppressor grid opposite the radially aligned gaps in the control .25 grid and the screen grid is made greater than at other portions of the suppressor grid. I prefer to obtain this increased shielding of the grid gaps by making the suppressor grid of closer mesh or smaller pitch opposite the screen grid gaps than 30 elsewhere, and in this way introduce between the anode and the more open portions or gaps of the screen grid a shield at the potential of the suppressor grid, and preferably as wide or wider than the gaps. In this way my improved tube .35 may be made to have substantially the same grid plate capacitance as the conventional variable mu pentode, and also a higher ratio of control grid-plate transconductance to plate current.

The novel features which I believe to be char- .4 acteristic of my invention are set forth with particularity in the appended claims, but the invention itself will best be understood by reference to the following description taken in connection with the accompanying drawing, in which one 5 embodiment of the invention in an electrode assembly for a pentode is illustrated schematically.

Referring to the drawing, the electrode assembly illustrated comprises an indirectly heated cathode l of the conventional oxide-coated type 50 with a conventional heater, an anode 2, and three grids interposed between and coaxial with the cathode and the anode. The control grid 3 nearest the cathode has, for purposes of illustration, agap or open portion near the middle, the screen 55 grid 4 has in the middle a gap aligned with and substantially as wide or wider than the gap in the control grid 3. I prefer to have not only the gaps but also the conductors or grid wires of the two grids in substantial alignment. The suppressor grid 5 between the screen grid and the anode is of uniform pitch or mesh, except at the middle portion where a close winding 6 forms a shielding electrode opposite the gaps in the control and screen grids. This closely wound portion 6 of the suppressor grid is to advantage wide enough to practically cover the gap in the screen grid, and is small enough pitch to act as a shield between the anode and the gaps in the control grid and the screen grid, but not small enough to produce a potential minimum between it and the screen grid. The pitch of the close winding 6 is preferably so chosen that the grid to plate capacitance is substantially the same as in a conventional variable mu pentode of similar type and construction with a screen grid of uniform pitch throughout its length.

In the particular embodiment illustrated in the drawing the control grid and the screen grid are of uniform pitch throughout their length except for the more open portions or gaps at the middle of the grids, and the suppressor grid is also of uniform pitch throughout its length except for the more closely wound portion 6 at the middle of the suppressor grid opposite the gaps in the control grid and the screen grid. The suppressor grid is maintained at a constant potential, preferably by a connection to the cathode inside the tube as in the conventional pentode, and consequently the closely wound middle portion of the suppressor grid is effectively a shielding electrode for the gaps in the control and screen grids. It is possible by the use of such a non-uniform suppressor grid, which is of closer mesh near the middle than at other portions, to modify the characteristics of the tube and obtain useful alterations in certain characteristics of the tube.

In one practical embodiment of my invention in a variable mu pentode of the small high frequency type I obtained good results with grids of the helical or wound type in which the grid wires were about 1 mil, or more exactly .0012 inch in diameter, the control grid being conventional with 190 turns per inch with the exception of a gap of two turns at the middle, the screen grid being similar to the control grid except that it had 200 turns per inch and a gap at the middle of four turns, while the suppressor grid was of 60 turns per inch throughout its length except for a closely wound middle portion of 6 close spaced turns of turns per inch. This tube when tested by conventional test methods had plate current of 5.4 m. a. and a control grid- -plate transconductance of about 1900 micromhos, so that the ratio of control grid-plate transconductance to plate current was higher than in a corresponding conventional variable mu pentode, while the grid-plate capacitance was only about .007 micromicrofarad, substantially the same as in a corresponding conventional variable mu tube. In this tube the removal of the suppressor grid and of the shielding effect due to the close spaced turns at the middle of the suppressor grid resulted in an increase of about 50% in gridplate capacitance.

I claim:

1. An electron discharge tube comprising a cathode, anode, and three concentric grids interposed between said cathode and anode to be passed in succession by the electron stream from said cathode to said anode, each of the two inner grids near the cathode having intermediate its ends a portion of more open mesh than the remaining portions, the open portions of said two inner grids being substantially in alignment between said cathode and said anode, and the third grid having opposite the more open portions of said two inner grids a portion of closer mesh than the more open portion of said inner grids and the remainder of said third grid.

2. An electron discharge tube comprising a cathode, anode, and three helically wound concentric grids coaxial with and interposed between said cathode and anode to be passed in succession by the electron stream from said cathode to said anode, the two inner grids near the cathode being of substantially the same variable pitch and with their more open mesh portions in substantial alignment radially of the cathode to provide aligned gaps in said inner grids,-the third grid having opposite said aligned gaps a part of closer mesh than said more open mesh portions of said inner grids to form an electron shield between said anode and. the more open'mesh portions of said two inner grids.

3. An electron discharge tube comprising a cathode, anode, and. three helically wound variable pitch concentric grids coaxial with and interposed between said cathode and anode to be passed in succession by the electron stream from said cathode to said anode, the two inner grids nearer the cathode being of substantially the same pitch and having their large pitch portions substantially in alignment radially of said cathode, and the third grid next the anode having 0pposite the large pitch portions of said two inner grids a portion of smaller pitch than the remain-- der of said third grid and the large pitch portion of said inner grids.

4. An electron discharge tube comprising a cathode, an anode, and three concentric grids interposed between said cathode and anode to be passed in succession by the electron stream from said cathode to said anode, the conductors of each of the two inner grids near the cathode being spaced non-uniformly along said grids with the more widely spaced conductors of said two inner grids substantially in alignment between said cathode and anode to provide aligned gaps in said inner grids, and the conductors of the third grid being spaced non-uniformly with the more closely spaced conductors opposite the gaps in said inner grids and spaced closer than said more widely spaced conductors of said inner 4 grids.

5. An electron discharge tube comprising a cathode, an anode, and three concentric helically wound grids coaxial with and interposed between said cathode and anode to be passed in succession by the electron stream from said cathode to said anode, each of the two inner grids near the cathode having two sections of uniform pitch spaced to leave between the adjacent ends of said sections a gap wider than the spacing between turns of said sections and with said gaps opposite one another, the third grid having two sections of uniform pitch opposite the uniform pitch sections of said inner grids and a middle section opposite the gaps in said inner grids and of smaller pitch than the pitch of said inner grids at said gaps.

6. An electron discharge tube comprising a cathode, an anode, and three helically wound grids of different diameters coaxial with and sur- Val rounding said cathode, the first grid from the cathode being a control grid having intermediate its ends a gap of greater pitch than the remainder of said control grid, the second grid being a screen grid having opposite the gap in said control grid a gap of greater pitch than the remainder of said screen grid, the gaps of said control grid and of said screen grid being in substantial alignment radially of the cathode, and the third grid being connected to said cathode and having opposite the gap in said screen grida section of substantially the width of said gap and of a pitch smaller than the pitch of said screen grid at said gap and of the remainder of said third grid.

DON G. BURNSIDE. 

